Williams was apprehended Tuesday after he was spotted by a Mexican citizen who had seen the fugitive's photo in a newspaper, said Bill Lewis, FBI assistant director in charge in Los Angeles.

Williams, who fled Los Angeles in 2011, was taken into custody by Mexican officials in the Caribbean town of Playa del Carmen in the state of Quintana Roo, authorities said.

Williams "was actively writing and publishing down there" and was "not living under an assumed name," Lewis said.

"We had an idea he may have fled to Mexico," Lewis said. But he added that the exact area was unknown until the witness contacted authorities.

At least 10 alleged victims between the ages of 9 and 17 were identified by authorities, according to the FBI. Many live in Third World countries. Williams has traveled extensively and lived in Southeast Asia and Polynesia.

A four-count federal indictment filed April 30 alleges crimes involving two 14-year-old boys that Williams met online in 2010. He allegedly "engaged in sexual activity via Internet webcam sessions with the boys and expressed a desire to visit them in the Philippines to have sex," the FBI said in a statement.

Williams went to the Philippines in January 2011, where he is suspected of committing "sexually explicit conduct" with the boys, took photos of the encounters and brought the photos back to Los Angeles County, the indictment said.

In 2011, the Los Angeles Police Department was alerted by an adult student that Williams was allegedly seeking out minors for sex through the Internet, Assistant Chief Michel Moore said. The accusation prompted an investigation in which Williams was interviewed by an LAPD detective, but there "was insufficient evidence for a warrant," Moore said.

"We did have our suspicions." Moore told the Los Angeles Times.

When Williams returned from his Philippines trip, the FBI seized his laptop computer, authorities said. Moore said the computer allegedly contained evidence of sex crimes with boys overseas and images of minors in sexual situations. By the time the evidence was recovered, Williams had fled Los Angeles, according to authorities.

Lewis said Wednesday that the FBI was only aware of victims overseas. But he added that authorities believe there may be other victims and urged them to come forward.

Williams taught anthropology, gender studies and history at USC, according to the university.